A direct melt process and a marble melt process have been used in the production of glass fibers as typical melting methods for forming a plurality of glass fibers by fiberizing a molten glass, gathering the glass fibers, and spinning them as glass fiber yarn. In the direct melt process, powders or particulate matters of various mineral matters are mixed as glass raw materials, injected into a melting furnace, and melted to produce a molten glass. On the other hand, in the marble melt process, glass gobs referred to as marbles prepared by solidifying a molten glass are remelted to produce a molten glass (for example, see Patent Literature 1).
For spinning glass fibers, solid raw materials such as glass raw materials of mineral matters or glass gobs are melted as described above. However, bubbles are unfavorably generated during melting. The generated bubbles may be bubbles caused when raw materials are injected into a melting furnace, bubbles of volatile components (carbonate, sulfate, oxide) contained in the raw materials, bubbles generated at an interface of a zirconia brick of the melting furnace, or bubbles generated by reaction with platinum in the melting furnace.
Due to these bubbles, some bubbles may exist in glass fibers during subsequent spinning. When this happens, glass fiber yarn may be unfavorably cut during spinning or mechanical strength or electrical insulation of a molded article using spun glass fiber yarn as a reinforcement material may be reduced.
Thus, the bubbles are reduced by adding clarifying agent to the glass raw materials or solid raw materials or attaching a valve to an outlet of the melting furnace as disclosed in Patent Literature 1.